As a former MSFT employee (on Azure), Teams daily active users are counted even when someone on Outlook hosts a meeting, which is automatically converted into a Teams meeting link. Therefore, unless an enterprise is still sticking with Skype, Teams users will automatically get trickled in from Outlook.
That would only be if they hosted an online meeting (an audio or video conference), not for any outlook meeting (i.e. a meeting booked with outlook doing scheduling, but meeting happening in a good old meeting room). And that is then comparable to slack video conferencing, so probably a fair comparison.